Viroids offer new gene-silencing possibilities

They're not life as we know it, not even life at all. A viroid is just a naked, sub-microscopic loop of single-stranded RNA -- a virus in the buff. But despite their simplicity, viroids possess a primeval weapon that can devastate crops- a weapon that of recent and great interest to molecular biologists.

CSIRO Plant Industry molecular biologist Dr Ming-Bo Wang says viroids have evolved the ability to silence crucial genes in their hosts.

Dramatic symptoms like stunting, necrosis, and developmental defects in diseases like cadang cadang in coconut palms, avocado sunblotch, and potato spindle tuber, represent a loss-of function mechanism, mediated by RNA interference (RNAi) or RNA silencing.

Wang, who is working on potato spindle-tuber viroid in collaboration with his colleagues in CSIRO and elsewhere, says these pathogens basically work by mirroring plant gene sequences.

Their single-stranded RNA sequences undergo replication to make complementary copies, which base-pair with the master copies to form double-stranded RNA (dsRNA) molecules.

In a defence response, the plant's inbuilt virus-busting ribonuclease system detects the double-stranded RNA complex and cleaves it, generating small fragments called small interfering RNAs or siRNAs. These siRNAs then guide ribonucleases to the homologous sequence in host gene mRNA and destroy the mRNA.

Wang says viroids tend to exist in long-lived crops; "huge armadas" of viroids replicate in plant cells, accumulating random errors - mutations - that converge upon the gene sequences of their hosts. Wang says it takes time to make a match, so viroid diseases are usually a problem only in longer-lived crops, including tree and other clonally propagated crops.

Fortunately, their virulence is self-limiting. Wang says natural selection eliminates the most dangerous variants because they kill their host plants.

Wang says a single nucleotide change in the genetic blueprint of the potato spindle tuber virus can convert it from a harmless to a virulent form.

Viroids are able to move easily between plant cells, and between the nucleus and the cytoplasm once inside a healthy cell. They are also extremely resistant to RNA silencing themselves, which Wang says is a property that has evolved to help viroids evade RNA silencing-mediated plant defence mechanism

In his paper at the ComBio2003 conference, to be held in Melbourne September 28 to October 2, Wang will describe, his evidence for the involvement of RNA silencing in mediating viroid symptom and evolution.

Conference web site: http://www.asbmb.org.au/combio2003